This invention relates to load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners.
Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only.
A popular method for testing and characterizing such microwave and millimeter-wave transistors in the non-linear region of operation is “load pull” (FIG. 1). Load pull is a measurement technique employing impedance tuners 2, 4 and other test equipment, such as signal sources 1, test fixtures and DUT 3 and power meters 5, the whole controlled by a computer 6; the computer controls and communicates with the tuners 2,4 and other equipment 1, 5 using digital cables 7, 8, 9. The tuners are used in order to manipulate the microwave impedance conditions in a systematic and controlled manner under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); tuners allow determining the optimum impedance conditions for designing amplifiers and other microwave components for specific performance targets, such as gain, efficiency, inter-modulation etc.; this specification refers hence to “tuners” as being “impedance tuners”, in order to distinct from “tuned receivers (radios)”, commonly referred to as “tuners”, because of the included tuning circuits (see ref. 2). Insertion loss between the DUT and the tuners 10, 11 reduce the tuning range (FIG. 3). It is a general aim in the technology to minimize this insertion loss 10, 11. This can be done by physically approaching the tuning core of the tuner (i.e. the tuning probe (slug)) to the DUT.
Impedance tuners consist, in general, of a transmission line with center conductor 23 and, one or more (see ref. 4) adjustable tuning probes 22, FIG. 2; the probe (slug) 22 is attached to a precision vertical axis 21, which is mounted in a mobile carriage 28; the axis 21, controlled by motor 27 can move the probe 22 vertically 216 in Y direction, starting at the top and moving towards the center conductor 23; the carriage 28 can move the probe 22 horizontally 217 (in X direction) either towards or away from the test port 25 (and the DUT, which is attached to the test port) parallel to the center conductor 23. The vertical movement 216 changes the amplitude of the reflection factor, seen at the tuner test port 25, whereas the horizontal movement 217 changes its phase. This way a large portion of Smith chart is covered allowing quasi-infinite impedances from a minimum value Zmin to a maximum value Zmax to be synthesized at any given frequency within the “tuning range” of the tuner. Typical values of state of the art tuners are |Zmin|=2Ω and |Zmax|=1250Ω; this corresponds to a Voltage Standing Wave Ratio (VSWR) of 25:1. The relation between reflection factor and impedance is given byGAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo)  {eq.1},wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. |GAMMA| varies between 0 and 1; a typical value used for Zo is 50Ω (see ref. 3). The higher |GAMMA| the higher the “tuning range”, see FIG. 3. The equivalent, offering higher reading resolution both for low and high |GAMMA| is the Voltage Standing Wave Ratio:VSWR=(1+|GAMMA|)/(1−|GAMMA|)  {eq.2}which varies between 1 and infinite.
Metallic probes 22, 41 or “slugs” are made in a parallelepiped form 41 with a concave bottom, which allows to capture, when approaching the center conductor 43, the electric field which is concentrated in the area between the center conductor 43 and the ground planes of the slabline 44 (FIG. 4). This “field capturing” allows creating high and controllable reflection factors. The critical part is the required proximity [S] and accuracy of both the vertical 46 and horizontal 45 probe movement, whereby changes in the vertical probe position 46 of a few micrometers affects the VSWR by a large extent. This invention discloses a passive single and multi-probe tuner structure maximizing the tuning range up to 110 GHz.